No full screen API so impossible to make lots of types of game experiences.

No orientation API so impossible to make games and other experiences that require a certain orientation

No WebXR (though Apple will allow it on Vision Pro)

No support for ResizeObvserver devicePixelContentBoxSize so impossible to get correct rendering reguardless of user's zoom level.

No simple PWA installation. Requires an obscure incantation that only expert users know.

That's just a few off the top of my head.

Yes, I know all the comments will be about how they don't want those features. That's really irrelevant. Allow them to be turned off. Require permissions. Those features have been shipping on other OSes, Desktop and Mobile for > 5 years and the world hasn't ended.